1. Technical Field
The present invention relates to a printed wiring board that has an electromagnetic band gap (EBG) structure and a method of producing the printed wiring board.
2. Background
To artificially control the frequency dispersion of electromagnetic waves, structures in which conductor patches or other conductors are arrayed at regular spacings have been proposed. Among these structures, an electromagnetic band gap (referred to below as an “EBG”) structure has been used for noise reductions, measures against interferences, and other similar purposes in a printed wiring board or a device package substrate. This EBG structure has an ability to reduce the propagation of electromagnetic waves in a specific frequency band. Examples of the EBG structure which have been proposed in the art include: a mushroom EBG structure having mushroom-shaped conductors; and a via-less EBG structure having no vias.
A typical multilayered printed wiring board is designed such that solid patterns having low impedance are formed on a power supply layer and a GND layer in order to stabilize an applied voltage and a supplied current. If both a digital circuit and an analog circuit are connected to the same power supply, not only DC components but also high-frequency AC components may be transmitted in typical solid patterns. In this case, high-frequency components generated in the digital circuit is prone to being transmitted to the analog circuit as noises. More specifically, the power supply layer and the GND layer have a resonance frequency depending on their shapes, and an electromagnetic wave whose frequency is the same as this resonance frequency becomes a standing wave. As a result, an electromagnetic wave with a specific frequency is transmitted exceptionally well. In short, a problem with multilayered printed wiring boards is that when a high-frequency noise generated in an IC or other digital circuit coincides with the resonance frequency, the operation of an analog circuit may be affected by this noise.
The following suggestions have been taken against the above noise problem.
Japanese Unexamined Patent Publication No. 2008-131509 proposes an EBG structural body for a printed wiring board, in which slits are formed in the power supply layer. This EBG structural body can be formed only with the power supply layer and thus requires no complex members, such as vias.
Japanese Unexamined Patent Publication No. 2010-199881 proposes a waveguide structure having an EBG structure, in which the spiral wires of open stub structures are connected to a GND plane through vias. In this EBG structure, a transmission line is connected to a stub wire through a via. Thus, the block frequency can be determined to be the resonance frequency at which the input impedance of the open stub becomes substantially zero. Since the resonance frequency can be controlled by the length of the stubs, the occupied area of the stubs can be decreased by forming the stub wires into a spiral shape. Therefore, this EBG structure is suitable for a compact design.
Japanese Unexamined Patent Publication No. 2013-183082 proposes a multilayered printed wiring board in which a power supply is divided into multiple pieces having a patch shape. An open stub is disposed in a part of each divided piece. By forming the EBG only with the power supply layer, the EBG structure is applicable easily to a compact design.
The report titled “Study for Planar EBG Structure with Ferrite Thin Film for Practice Use”, which was written by Yoshitaka Toyota et al. at Okayama University and in Journal of 28th spring meeting in the Japan Institute of Electronics Packaging and was issued on Mar. 5, 2014, proposes noise measures using an EBG pattern with a meander line. By interconnecting patches (cells) with meander lines, the inductance of the EBG pattern can be increased. Thus, this EBG structure is intended for a compact design.
In the EBG structural body proposed in Japanese Unexamined Patent Publication No. 2008-131509, forming the slits in the solid pattern in the power supply layer is expected to reduce the transmission of high-frequency components. If the solid pattern is separated completely by the slit, the electricity cannot be supplied. Therefore, the solid patterns need to be partially continuous with power supply. To block noises in a 2.5 GHz band used for radio communication, for example, unit cells each having a size of about 16.5 mm per side are required. Unfortunately, such large-sized unit cells are not applicable easily to portable electric devices, such as notebook PCs.
The waveguide structure proposed in Japanese Unexamined Patent Publication No. 2010-199881 involves additionally forming a layer in which the open stub EBG structure is to be formed. Furthermore, since the wires of the open stubs are connected to the GND plane through the vias, a step dedicated to forming the vias needs to be added to the producing process of the printed wiring board. The addition of the dedicated step results in the increase in the number of steps, possibly pushing up an overall cost.
In the multilayered printed wiring board proposed in JP 2013-183082 A, the cells need to be interconnected by wide wires in order to supply a desired amount of current. If a large number of cells are used to widen the cutoff frequency band as in Example shown in FIG. 3, a complex current route may be created. In which case, the current consumption might be difficult to calculate.
In the report titled “Study for Planar EBG Structure with Ferrite Thin Film for Practice Use”, which was written by Yoshitaka Toyota et al. at Okayama University and in Journal of 28th spring meeting in the Japan Institute of Electronics Packaging and was issued on Mar. 5, 2014, the cells are interconnected by the meander lines. Using the meander lines is expected to increase the inductance component and thus downsize unit cells. However, if a larger amount of current is supplied, wider meander lines need to be used, because the width of a meander line depends on its allowable current. According to Example, cells each having a size of about 7 mm per side are required to take the measures against noises in a 2.5 GHz band. Consequently, unit cells may be difficult to downsize.